1. Field of the Invention
The present invention relates to a rotary electric machine provided with a stator having slots formed at a ratio of two slots per phase per pole and a rotor for generating a magnetic flux with a field winding and a permanent magnet.
2. Description of the Related Art
Conventional automotive alternators are provided with: a stator in which a stator winding is mounted to a stator core, and a rotor for generating a magnetic flux with a field winding and permanent magnets. In this stator core, slots are formed at an even angular pitch at a ratio of two slots per phase per pole. The stator winding is constructed by three-phase zigzag-connecting windings installed in the stator core. A solid core is used for the magnetic field poles, and a field winding is added to a central axis to construct a claw-pole rotor. (See Patent Literature 1, for example.)
Patent Literature 1: Japanese Patent Laid-Open No. HEI 9-154266 (Gazette: FIG. 22)
In conventional automotive alternators, eddy current loss is generated by harmonics at the rotor surface due to stator magnetomotive force harmonics arising due to stator winding currents and permeance harmonics in an air gap caused by stator slots (hereinafter called “slot harmonics”) giving rise to poor efficiency, and also to deficiencies in performance such as temperature increases in the rotor and bearings, etc. Since the rotor is solid, the eddy current loss at the rotor surface is large.
Thus, one disadvantage has been that heat generated by this eddy current loss at the rotor surface may give rise to thermal demagnetization of the permanent magnets disposed between the claw-shaped magnetic poles.
Because permanent magnets are provided in the rotor, an induced voltage accompanying rotation of the rotor is generated in the stator winding even in a field de-energized state. Thus, another disadvantage has been that in a high-speed rotation region, this induced voltage may exceed a power supply voltage and an element breakdown voltage.
In addition, if a conventional automotive alternator is used as an electric motor by performing inverter control, it is necessary to establish a voltage limit based on a breakdown voltage of semiconductor elements constituting the inverter. Thus, if permanent magnets are provided in the rotor, the induced voltage occurring in the stator winding must be controlled in response to the rotational frequency so as not to exceed the voltage limit. Since this induced voltage will seriously affect the elements if it exceeds the voltage limit even for an instant, instantaneous overvoltages due to harmonics in the induced voltage generated by the permanent magnets are also considered a problem. Consequently, it is desirable for the permanent magnets to be constructed such that their harmonic components are reduced.